(1) One of the conventional electric tackers is disclosed in Japanese Patent Application No. 58-249626 (published under No. 60-135182). The electric tacker disclosed therein includes a primary switch for closing or opening the circuit connecting a battery and an electric motor and a secondary switch (cam switch) connected to the primary switch in series to ensure that the tacker discharges only one shot each time a trigger is depressed. A hammer for driving a tack is provided with a horizontal projection which is adapted to engage with a driving cam. The driving cam is operated by the motor. Initially the hammer is in a bottom dead center, and the driving cam is in such an orientation that the driving cam can engage with the horizontal projection of the hammer immediately after the driving cam is operated. When the trigger is depressed, the primary switch is actuated so that the driving cam starts to rotate and engages with the horizontal projection of the hammer to raise the hammer from the bottom dead center to a top dead center. When the hammer has reached the top dead center, the driving cam disengages from the hammer with the result that the hammer is pushed down to the bottom dead center by the action of a spring. The hammer thus drives a tack into an object to be tacked. When the hammer has returned to the bottom dead center, the secondary switch is activated to stop the motor. Then, the operator releases his hold on the trigger. Then, the motor starts again to reorientate the driving cam such that the driving cam can engage with the hammer instantly the operator depresses the trigger again. Then, the secondary switch is deactivated to stop the motor again.
This conventional tacker has the following drawbacks: [I] Initially the hammer is in the bottom dead center, or in its lowest position. Therefore, the operation of depressing the trigger does not cause the hammer to drive a tack instantly. If the tacker is modified to start down from the top dead center to drive a tack on the bottom dead center and return to the top dead center, the operator can drive a tack instantly he triggers the tacker.
[II] Also, it appears that the hammer head initially is located out of the muzzle section since the hammer initially is in the bottom dead center. Thus, it is difficult to press the muzzle section properly against the required position on an object to be tacked. In addition, it is possible that, for the same reason, the hammer head may damage the object to be tacked when the muzzle section is pressed against the object.
[III] Moreover, since the hammer starts up, the hammer not only is unable to drive a tack, but also returns down to the bottom dead center if the battery happens to run down when the hammer is rising to the top dead center.
[IV] Furthermore, as described above, after the operator has released his hold on the trigger, the motor starts again to reorientate the driving cam. That is, making one shot involves applying an electric current to the motor twice. The electric current consumed to reorientate the driving cam is a waste of energy.
[V] The necessity to use the two switches, primary and secondary, makes the tacker a costly construction. It is possible to produce an electric tacker with only one switch.
[VI] Also, since the driving cam lifts up the hammer from the bottom dead center to the top dead center by making a substantially half rotation, a large torque is required. In addition, the necessity to provide a space in which the driving cam can make such a rotation makes it impossible to produce a compact tacker.
(2) Another conventional electric tacker is disclosed in Japanese Utility Model Application No. 60-172074 (published under No. 62-81581). This Japanese application has a corresponding U.S. Pat. No. 4,724,992. The electric tacker disclosed therein has a switch plate with a forward end which is initially in a recess in a hammer cam and a connector plate with an operating projection which is initially in an offset portion of the hammer cam. The connector plate also has a catch projection which is initially in engagement with a connector. The connector is also in engagement with the switch plate. A rearward end of the switch plate is in contact with a push button of a switch for an electric motor. Initially the hammer is in top dead center. When a trigger is depressed, the connector plate is moved rearward to move the switch plate rearward. Hence, the forward end of the switch plate disengages from the recess in the hammer cam, and the hammer is pushed down by the action of a spring. The hammer thus drives a tack into an object to be tack, on a bottom dead center. At the same time that the forward end of the switch plate disengages from the recess, the push button is depressed by the rearward end of the switch plate to start the motor. The motor operates a driving member. A worm wheel is in engagement with the driving member, and is provided with engaging projections. The worm wheel is rotated by the driving member. When the hammer is pushed down, the operating projection of the connector plate disengages from the offset portion of the hammer cam since the offset portion moves down. When the hammer has been pushed down, one of the engaging projections of the worm wheel comes into engagement with the bottom of the hammer and returns the hammer to the top dead center. Then, the forward end of the switch plate engages with the recess of the hammer again. At the same time, the push button is released. Then, the operator releases his hold on the trigger. Since the operating projection of the connector plate does not engage with the offset portion of the hammer cam again (and, hence, the operating projection of the connector plate does not engage with the connector again) unless the operator releases his hold on the trigger, there is no possibility that the operator may make two shots by depressing the trigger once.
Unlike the hammer of the first-mentioned tacker, the hammer of this conventional tacker starts from the top dead center drive a tack on the bottom dead center and returns to the top dead center. This conventional tacker has an advantage over the first-mentioned tacker in this respect.
However, this tacker has the following drawbacks:
[I] As understood from the foregoing description, the hammer of this tacker is not electrically pushed down. As described above, when the operator moves the switch plate reward through the connector plate by depressing the trigger. By so doing, the operator disengages the forward end of the switch plate from the hammer cam to push the hammer down. Hence, depressing the trigger does not start down the hammer lightly and quickly. The motor is not used to start the hammer, but used to raise the hammer from the bottom dead center.
[II] If the battery happens to run short of electricity when the hammer is being raised from the bottom dead center, the hammer stops, but does not return to the bottom dead center since the worm wheel is in engagement with the driving member. This tacker has an advantage over the first-mentioned tacker in this respect. However, in such an event, if the operator releases his hold on the trigger, the push button of the switch remains depressed by the rearward end of the switch plate. Hence, the cells of the battery and the motor make a closed circuit. Therefore, if the amounts of electricity that remains in the cells are different, the cell where the smallest amount of electricity remains discharges an excessive amount of electricity and electrolysis occurs in that cell. Thus, that cell is no longer capable of being used.
(3) A different electric tacker is disclosed in Japanese Patent Application No. 62-189984 (published under No. 63-174882). This application has a corresponding U. S. Pat. No. 4,807,793. The electric tacker disclosed therein includes a rod which is normally in alignment with, but is away from, a push button of a switch for an electric motor. The tacker also includes a gear with a toothed segment which can engage with a hammer. The gear is also provided with a release pin. Initially the hammer is in a bottom dead center. When a trigger is depressed, the rod depresses the push button, with its lower end, to rotate the gear. Thus, the toothed segment of the gear comes into engagement with the hammer to raise the hammer until the hammer reaches a top dead center. Then, the toothed segment disengages from the hammer and, hence, the hammer is pushed down from the top dead center by the action of a spring to drive a tack. At the same that the hammer reaches the top dead center, the release pin of the gear engages with a central portion of the rod to disengage the lower end of the rod from the push button. The push button is thus released to switch off the tacker. The hammer is in the bottom dead center. Then, the operator releases his hold on the trigger. The rod thus returns to the upper position, and is returned, by the action of a return spring, to the initial position where the rod is aligned with the push button of the switch again.
This conventional tacker has the following drawbacks:
[I] The tacker has the same disadvantage as the first-mentioned tacker, namely, the disadvantage that the hammer starts up from the bottom dead center to drive a tack.
[II] It is a superficial view that initially the toothed segment of the gear is shortly before its engagement with the hammer. It should be said that, only when the toothed segment is shortly before its engagement with the hammer, the toothed segment comes into engagement with the hammer instantly the trigger is depressed. That is, when the toothed segment has raised the hammer to the top dead center and has disengaged from the hammer, the push button is released to stop the gear. Hence, the toothed segment is stopped in a position far away from the initial position. Thus, when the trigger is depressed to make a next shot, the toothed segment does not engage quickly with the hammer. This disadvantage, coupled with the first-mentioned drawback, results in a very slow response.
[III] In addition, if the motor stops, whether by a shortage of the electricity in the battery or by an insufficient depression of the trigger, and the gear stops when the rod is between the release pin and the return spring, the rod does not return to the initial position if the operator releases his hold on the trigger and instead the rod is fixed in the position between the release pin and the return spring where the rod is out of alignment with the push button of the switch. Thus, the push button of the switch cannot be depressed if the trigger is depressed again. In such a case, it is necessary for the operator to rotate the gear manually to disengage the release pin of the gear from the rod so that he can operate the tacker again.
[IV] Moreover, the tacker has the same disadvantage as the first-mentioned tacker, namely, the disadvantage that a large torque is required since the gear lifts up the hammer from the bottom dead center to the top dead center by making a substantially half rotation. Also, as with the first-mentioned tacker, the necessity to provide a space in which the gear can make such a rotation makes it impossible to produce a compact tacker.